The invention relates to a solar thermal power station and a method of operating a solar thermal power station.
Among renewable forms of energy generation, the solar thermal power station concept is said to have the greatest potential to be able in the future to generate a large proportion of the worldwide demand for electric energy without producing emissions and at competitive production costs. Not least, the Desertec or DII GmbH project focuses on erecting solar fields having a size of many square kilometers on the North African continent and in the Middle East, which are to be operated by parabolic trough mirrors and/or Fresnel mirrors arranged in a cascade-like manner to generate solar thermal energy for Europe. In such plants, a heat transfer fluid circulates in special, kilometer-long absorber tubes which are heated to process temperatures of up to 600° C. by bundling/focusing of solar radiation by the mirror geometries mentioned. A steam-driven turbine is operated by a heat exchange process and electric energy is released. The heat transfer fluid (“HTF”) which has been cooled thereby is available in a cyclic process for renewed heating by thermal energy. Every solar thermal plant therefore has two HTF tanks, one containing cooled HTF and one containing hot HTF.
The HTF has to meet demanding requirements since its property parameters such as melting point, heat capacity, thermal conductivity, viscosity and specific gravity dictate the final power generation costs and thus determine the competitiveness and cost parity compared to conventional energy generation.
The melting point of the HTF used is of particular importance. The heat transfer media used at present are organic or inorganic in nature. The first generation of the HTFs used for this purpose are organic heat transfer oils, with the best-known representative for solar thermal applications being a eutectic-melting mixture of 26.5% by weight of biphenyl and 73.5% by weight of diphenyl ether (e.g. Therminol® VP-1 from Solutia™) which solidifies at 12° C. However, owing to the organic nature of the molecules, such mixtures quickly generate vapor pressures of up to 15 bar at high temperatures, so that the maximum operating temperature of this mixture known as “VP-1” is limited to below 400° C. since above these process temperatures thermal degeneration of the organic structures also occurs.
Since efficiencies of power stations are known to increase overproportionally with the process temperature, the use of such organic HTFs stands in the way of economical, solar thermal energy generation in the very high MWe power range. In addition, such a preparation poses a risk to human beings, animals and nature in the event of a leakage.
For this reason, attention has for some decades focused on the use of inorganic salt eutectics having low melting points as heat transfer medium. The negligible vapor pressure of liquid salts or mixtures thereof, which allows virtually atmospheric-pressure operation in the absorber pipes, has been found to be particularly advantageous. Furthermore, salt melts have a thermal conductivity or specific heat capacity which is greater by a factor of from two to three and allows the solar energy which can be introduced per unit volume to be increased greatly relative to organic heat transfer oils. The best-known representative of an inorganic salt mixture used for this purpose is “solar salt”, viz. a mixture of sodium nitrate (60% by weight) and potassium nitrate (40% by weight).
This nontoxic preparation which is a factor of four cheaper than heat transfer oil VP-1 has a solidus temperature of about 240° C. To reduce the melting point, ternarization of the mixture with calcium nitrate (solidus temperature of the eutectic: 133° C.) or additional quaternization with lithium nitrate (solidus temperature of the eutectic: 97° C.) is useful. However, the latter method in particular requires the use of the expensive lithium nitrate which is not adequately available globally in multiton quantities.
Since a melting point of less than 150° C. of the heat transfer medium used in solar thermal power stations is indispensable for economical operation, the interest in inorganic salt eutectics having solidus temperatures below 100° C. is very great since night operation per se does not generate any electricity. The measures for avoiding “freezing” of a salt mixture in the usually kilometer-long feed pipe systems and receiver systems during the night when the HTF is not heated by the sun are accordingly energy-consuming.
In general, the solar thermal power station is operated at night using external heating, with a not inconsiderable amount of the power generated during the day being consumed again so that the pipes do not burst as a result of the volume expansion of the solidifying salt and/or so that the plant does not have to be started up afresh in the morning.